Regulated power supplies are found in virtually all electronic devices, including battery chargers, cellular telephones, computers, computer monitors, televisions, audio equipment, and video cameras. One typical power supply, the DC to DC converter, operates from a DC source, generates an alternating current as an intermediate process, and delivers direct current to a load. Switched DC converters, like linear, transformer-based power supplies, deliver a regulated output and provide isolation between input and output circuits. Unlike a linear power supply, however, the transformers in a switched DC converter operate at much higher frequencies, as high as several megahertz. This permits the use of small components, including transformers and capacitors, while still providing for complete isolation between the input and the output.
The recent demand for efficient power supplies in consumer electronic equipment has resulted in various improvements to the basic DC converter design. For example, television and computer monitors typically include power supplies capable of operating in multiple modes. A switched DC converter operating in standby mode switches at a fixed lower frequency and dissipates less power than a converter operating in a run mode. In standby mode, only the essential devices, such as microprocessors or microcontrollers, are powered.
Known designs and techniques for multi-mode power supplies are inadequate to achieve the low power dissipation requirements of modern consumer electronics. The inadequacies of current designs are magnified when the power supply must operate over a wide range of line voltages and must provide a wide range of output voltages. Modern televisions, for example, require power supplies operable from line voltages ranging from approximately 85 VRMS to 260 VRMS, and operable to deliver regulated outputs from 5 volts to 133 volts. In standby mode, power supplies will be required to dissipate less than 2 watts. Current designs cannot cost-effectively meet these voltage range requirements while achieving acceptably low power dissipation.